1. Field of the Invention
The present invention relates to the field of earth boring tools and in particular relates to diamond cutters used on rotary bits.
2. Description of the Prior Art
Rotating diamond drill bits were initially manufactured with natural diamonds of industrial quality. The diamonds were square, round or of irregular shape and fully embedded in a metallic bit body, which was generally fabricated by powder metallurgical techniques. Typically, the natural diamonds were of a small size ranging from various grades of grit to larger sizes where natural diamonds of 5 or 6 stones per carat were fully embedded in the metal matrix. Because of the small size of the natural diamonds, it was necessary to fully embed the diamonds within the matrix in order to retain them on the bit face under the tremendous pressures and forces to which a drill bit is subjected during rock drilling.
Later, the commercial production of synthetically produced diamond grit and polycrystalline stones became a reality. For example, synthetic diamond was sintered into larger disk shapes and were formed as metal compacts, typically forming an amalgam of polycrystalline sintered diamond and cobalt carbide. Such diamond tables are commercially manufactured by General Electric Company under the trademark STRATAPAX. The diamond tables are bonded, usually within a diamond press to a cobalt carbide slug and sold as an integral slug cutter. The slug cutters are then attached by the drill bit manufacturers to a tungsten carbide slug which is fixed within a drill bit body according to the design of the bit manufacturer.
However, such prior art polycrystalline diamond (PCD) compact cutting slugs are characterised by a low temperature stability. Therefore, their direct incorporation into an infiltrated matrix bit body is not practical or possible.
In an attempt to manufacture diamond cutting elements of improved hardness, abrasion resistance and temperature stability, prior art diamond synthesizers have developed a polycrystalline sintered diamond element from which the metallic interstitial components, typically cobalt, carbide and the like, have been leached or otherwise removed. Such leached polycrystalline synthetic diamond is manufactured by the General Electric Company under the trademark GEOSET, for example 2102 GEOSETS, which are formed in the shape of an equilateral prismatic triangle 4 mm on a side and 2.6 mm deep (3 per carat), and as a 2103 GEOSET shaped in the form of an equilateral triangular prismatic element 6 mm on a side and 3.7 mm deep (1 per carat). However, due to present fabrication techniques, in order to leach the synthetic sintered PCD and achieve the improved temperature stability, it is necessary that these diamond elements be limited in size. Therefore, whereas the diamond compact slug cutters, STRATAPAX, may be formed in the shape of circular disks of 3/8" (9.5 mm) to 1/2" (12.7 mm) in diameter, the leached triangular prismatic diamonds, GEOSETS, have maximum dimensions of 4 mm to 6 mm. It is well established that the cutting rate of a diamond rotating bit is substantially improved by the size of the exposed diamond element available for useful cutting. Therefore, according to the prior art, the increased temperature stability of leached diamond products has been achieved only at the sacrifice of the size of the diamond elements and therefore the amount of diamond available in a bit design for useful cutting action.
What is needed then is a PCD cutter which is characterised by the temperature stability and characteristics of leached diamond products, and yet has the size available for useful cutting action which is characterised by the larger unleached diamond products.